Magnetizable device

ABSTRACT

A composition that includes a plurality of uniformly sized ferri- or ferromagnetizable particles, each particle having a largest dimension no greater than about 100 nm and being at least partially encased within an organic macromolecule, is disclosed herein.

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. Ser. No. 09/730,117,filed on Dec. 5, 2000, which is a continuation of U.S. Ser. No.09/308,166, filed on Jun. 25, 1999, which is a national stageapplication of International Application No. PCT/GB97/03152, filed Nov.17, 1997, which claims priority to Great Britain application No.9623851.4, filed on Nov. 16, 1996.

[0002] This invention relates to a magnetizable device which comprises amagnetic layer composed of domain-separated, nanoscale (e.g. 1-100 nm)ferromagnetic particles. The magnetizable device of the invention may beused as a magnetic storage device having improved data storagecharacteristics. In particular, the invention relates to magneticstorage media comprising single-domain, domain-separated, uniform,ferromagnetic nanoscale (e.g. 1-100 nm) particles which may be arrangedinto a regular 2-D packed array useful in the storage of information.

[0003] Among the possible pathways to ultrahigh-density (>=1 Gbit/in²)magnetic media is the use of nanoscale (1-100 nm) particles. Beyond thestandard requirements for magnetic media, a viable particulate mediashould have a small standard deviation in particle size as well as theparticles being exchange decoupled. These requirements are necessary toavoid adverse media noise. Current methods of fabricating nanoscaleparticles, such as arc-discharge or multiple target ion-beam sputtering,have not fully addressed these two requirements. Moreover, if theuniform particles are arranged into an ordered array, each particle canrepresent a “bit” of information at a predictable location furtherincreasing the media's efficiency. This invention details methods ofproducing particulate media that meet these requirements forultrahigh-density recording. This invention is also an open system whichallows for the production of a variety of magnetic materials, such thatthe media can be tuned for different applications.

[0004] In particular this invention details the use of an iron storageprotein, ferritin, whose internal cavity is used to produce thenanoscale particles. Ferritin is utilised in iron metabolism throughoutliving species and its structure is highly conserved among them. Itconsists of 24 subunits which are arranged to provide a hollow shellroughly 8 nm in diameter. The cavity normally stores 4500 iron(III)atoms in the form of paramagnetic ferrihydrite. However, thisferrihydrite can be removed (a ferritin devoid of ferrihydrite is termed“apoferritin”) and other materials may be incorporated. Examples includeceramics, superparamagnetic magnetite, acetaminophen, and even thesweetener aspartame. To address magnetic media concerns, the inventionincorporates ferromagnetically ordered materials.

[0005] According to a first aspect of the present invention, there isprovided a magnetizable device which comprises a magnetic layer composedof domain-separated, ferromagnetic particles each of which has a largestdimension no greater than 100 nm.

[0006] According to a second aspect of the invention, there is provideda magnetic recording medium which includes a magnetizable layer, whereinsaid magnetizable layer comprises a plurality of ferromagnetic particleseach having a largest dimension no greater than 100 nm, and each ofwhich particles represents a separate ferromagnetic domain. Themagnetizable layer is preferably supported on a non-magnetic substrate.

[0007] According to a third aspect of the present invention, there isprovided a magnetic composition comprising a plurality of ferromagneticparticles each of which is bound to an organic macromolecule, and eachof which has a largest dimension no greater than 100 nm. In this aspectof the invention, it is preferred that said organic macromolecule isferritin from which the normal core ferrihydrite has been removed andreplaced by a ferromagnetic particle.

[0008] As used herein, the term “ferromagnetic” embraces materials whichare either “ferromagnetic” and “ferrimagnetic”. Such usage is common inthe electrical engineering art.

[0009] The ferromagnetic particles used in the invention should be of amaterial and size such that they possess ferromagnetic properties atambient temperatures (e.g. 15° C. to 30° C.),

[0010] Preferably, the ferromagnetic particles each have a largestdimension no greater than 50 nm, more preferably less than 25 nm andmost preferably smaller than 15 nm. The largest dimension of theferromagnetic particles should not be so small that the particle willlose its ferromagnetic property and become superparamagnetic at thedesired operating temperature of the recording medium. Typically, foroperation at ambient temperature, this means that the magnetic particleswill normally be no smaller than about 3 nm in their largest diameter.

[0011] In the magnetizable device of the first aspect of this inventionand the magnetic recording medium of the second aspect of thisinvention, the distance between adjacent ferromagnetic domains ispreferably as small as possible to permit the maximum number of discretedomains in a given area, and provide the maximum storage capacity forthe recording medium. The actual lower limit will vary for differentmaterials and other conditions such as the temperature at which therecording medium is to be used. The key requirement, however, is thatneighbouring domains should not be able to interfere magnetically witheach other to the extent that the magnetic alignment of any domain canbe altered by neighbouring domains. Typically, the lower limit on thespacing of the domains is about 2 nm. The distance between adjacentdomains will be determined by the density of discrete domains required.Typically, however, to take advantage of the miniaturizationpossibilities provided by the invention, the distance between adjacentdomains will be no greater than 10 nm.

[0012] Generally the particles will be uniform in size, by which we meanthat the particles do not vary in largest diameter by more than about5%. One of the advantages of the use in the invention of an organicmacromolecule which binds a magnetic particle by surrounding it is thatthis can be used to select particles of a uniform size.

[0013] In the case where the particles are spheroidal, it will be thediameter of the particles which must be no greater than 100 nm.

[0014] In preferred embodiments of all aspects of this invention, eachferromagnetic particle is encased, or partially encased, within anorganic macromolecule. The term macromolecule means a molecule, orassembly of molecules, and may have a molecular weight of up 1500 kD,typically less than 500 kD. Ferritin has a molecular weight of 400 kD.

[0015] The macromolecule should be capable of binding by encasing orotherwise organising the magnetic particle, and may therefore comprise asuitable cavity capable of containing the particle; a cavity willnormally be fully enclosed within the macromolecule. Alternatively, themacromolecule may include a suitable opening which is not fullysurrounded, but which nevertheless is capable of receiving andsupporting the magnetic particle; for example, the opening may be thatdefined by an annulus in the macromolecule. For example, suitablemacromolecules which may be used in the invention are proteins, forexample the protein apoferritin (which is ferritin in which the cavityis empty), flagellar L-P rings, cyclodextrins, self-assembled cyclicpeptides. As an alternative to encasing the magnetic particles withinthe macromolecule, they may be organised on the macromolecule, such ason a bacterial S-layer.

[0016] Other materials which may be used in the invention to organisethe ferromagnetic particles are inorganic-silica networks such as MCMtype materials, dendrimers and micellar type systems.

[0017] The presently preferred macromolecule for use in the invention isthe apoferritin protein which has a cavity of the order of 8 nm indiameter. The ferri- or ferromagnetic particles to be accommodatedwithin this protein should have a diameter no greater than 8 nm.

[0018] The bound particles of this aspect of the present invention witha coating that inhibits aggregation and oxidation, also helping them tobe domain-separated.

[0019] In the magnetizable device of the first aspect of this inventionand the magnetic recording medium of the second aspect of thisinvention, the particles are preferably arranged in a 2-D ordered arraywhich would yield an ultrahigh-density magnetic media.

[0020] The ferromagnetic material may be a metal, such as cobalt, iron,or nickel; a metal alloy, such as an alloy which contains aluminium,barium, bismuth, cerium, chromium, cobalt, copper, iron, manganese,molybdenum, neodymium, nickel, niobium, platinum, praseodymium,samarium, strontium, titanium, vanadium, ytterbium, yttrium or a mixturethereof; a metal ferrite such as a ferrite containing barium, cobalt, orstrontium; or an organic ferromagnetic material.

[0021] When generating nanoscale particles, one major concern is thatthe particles produced are not superparamagnetic. Superparamagneticparticles are those which have permanent magnetic dipole moments, butthe moments' orientations with respect to the crystallographic axesfluctuate with time. This is not useful for a practical magnetic storagemedia. Superparamagnetism depends on the volume, temperature, andanisotropy of the particles. Via energy considerations, one can derivean equation relating these quantities. The volume at which a particlebecomes superparamagnetic (V_(p)) is given by: V_(P)=25 kT/K, where k isBoltzman's constant, T the temperature of the particle in degreesKelvin, and K the anisotropy constant of the material. Using thisformula, it is possible to determine the temperature at which a particlebecomes superparamagnetic (the “blocking temperature”) for a givenmaterial at a fixed volume. In our specific case, the fixed volume is 8nm in ferritin. If a cobalt metal particle with only crystallineanisotropy (that value being 45×10⁵) is a sphere with a diameter of 8nm, the blocking temperature is 353° K. This is within the range oftemperatures experienced within a hard disk drive, and the cobaltparticles may prove to be a useful storage medium. Obviously, there areother considerations such as the materials' coercivity, moment,saturation magnetisation, and relaxation time. By tuning the materialsincorporated into the ferritin, though, these can be addressed.

[0022] Ferritin is utilised in iron metabolism throughout living speciesand its structure is highly conserved among them. It consists of 24subunits arranged in a 432 symmetry which provide a hollow shell roughly8 nm in diameter. The cavity normally stores 4500 iron(III) atoms in theform of paramagnetic ferrihydrite.

[0023] However, this ferrihydrite can be removed (a ferritin devoid offerrihydrite is termed “apoferritin”) and other materials may beincorporated. The subunits in ferritin pack tightly, however there arechannels into the cavity at the 3-fold and 4-fold axes. Lining the3-fold channels are residues which bind metals such as cadmium, zinc,and calcium. By introducing such divalent ions one can potentially bindferritin molecules together, or at least encourage their proximalarrangement.

[0024] One method of preparing a 2-D packed array of ferromagneticallyordered particles of uniform size up to 8 nm includes the removal of theferrihydrite core from the native ferritin in aqueous solution, theincorporation of ferromagnetically ordered cobalt metal particles bysodium borohydride reduction of the aqueous Co(II) solution into theferritin cavities, the generation of a narrow size distribution throughultracentrifugation, the injection of particles into an MES/glucosesubphase solution upon which the 2-D array assembles, and the transferof the 2-D array to a substrate which is then carbon coated. In thismethod, the ferritin source may be a vertebrate, invertebrate, plant,fungi, yeast, bacteria, or one produced through recombinant techniques.

[0025] In the method described, a metal alloy core may be produced bysodium borohydride reduction of a water soluble metal salt. Otheroxidation methods include carbon, carbon monoxide, hydrogen, orhydrazine hydrate solution. Alternatively, a suitable-solution may beoxidised to yield a metal ferrite core. Oxidation may be chemical orelectrochemical to yield the metal ferrite.

[0026] In this method, other methods of selecting a narrow sizedistribution may be employed such as short or long column meniscusdepletion methods or magnetic field separation.

[0027] Further, in this method, divalent metal salts containing cadmium,calcium, or zinc may be added into the subphase solution to aid inparticle ordering.

[0028] Further, in this, other methods of arranging the particles into a2-D array may be employed, such as solution evaporation onto a solidsubstrate.

[0029] Further, in this method, the 2-D array may be coated withcarbon-based films such as hydrogenated or nitrogen doped diamond-likecarbon, or with silicon-based films such as silicon dioxide.

[0030] In the present invention, ferritin may be used to enclose aferromagnetic particle whose largest dimension is limited by ferritin'sinner diameter of 8 nm. The particles are produced first by removing theferrihydrite core to yield apoferritin. The is done by dialysis againsta buffered sodium acetate solution under a nitrogen flow. Reductivechelation using thioglycolic acid is used to remove the ferrihydritecore. This is followed by repeated dialysis against a sodium chloridesolution to completely remove the reduced ferrihydrite core fromsolution. Once the apoferritin is produced, ferr- or ferromagneticparticles are incorporated in the following ways. The first is byreducing a metal salt solution in the presence of apoferritin. This isperformed in an inert atmosphere to protect the metal particles fromoxidation which would lessen their magnetic benefit. A combination ofmetal salts in solution can also be reduced to generate alloys or alloyprecursors. Sintering or annealing in a magnetic field may be necessaryto generate the useful magnetic alloys. Another method is to oxidise acombination of an iron(II) salt and another metal salt. This gives ametal ferrite particle which does not suffer negatively from oxidation.The metal salts which are beneficial include salts of aluminium, barium,bismuth, cerium, chromium, cobalt, copper, iron, manganese, molybdenum,neodymium, nickel, niobium, platinum, praseodymium, samarium, strontium,titanium, vanadium, ytterbium, and yttrium.

[0031] A narrow size distribution of particles is necessary to avoidmedia noise. Such a distribution can be obtained through a variety ofprocedures including, but not limited to, density gradientcentrifugation or magnetic field separation.

[0032] While the production procedure detailed uses native horse spleenferritin, this invention should not be seen as limited to that source.Ferritin can be found in vertebrates, invertebrates, plants, fungi,yeasts, bacteria, or even produced through recombinant-techniques. Bycreating mutant apoferritins lacking the divalent binding site, othershave found that the mutant proteins assemble into oblique assemblies asopposed to the regular hexagonal close-packed.

[0033] While ferritin seems to be an ideal system for generatingnanoscale particles, it is not the only system available. For example,flagellar L-P rings are tubular proteins with an inner diameter of 13nm. By creating a 2-D array of these proteins, metal films could bedeposited into the tubular centres to create perpendicular rods ofmagnetic material. Also metal reduction in the presence of amicroemulsion can be used to generate nanoscale particles which arecoated with surfactant. This invention is open to other nanoscaleparticle production methods.

[0034] Finally an ordered arrangement of the particles is desired. Oneway to accomplish this is by injecting an aqueous solution of particlesinto an MES/glucose subphase solution contained in a Teflon trough. Theparticles spread at the air-subphase interface, and a portion denatureto form a monolayer film. The 2-D arrangement of encased particlesoccurs underneath this monolayer. After 10 minutes at room temperature,the arrangement and monolayer are transferred to a substrate by placingthe substrate directly onto the monolayer for 5 minutes. Afterwithdrawing the substrate, the attached arrangement is coated with athin layer of carbon for protection. Other methods such as solutionevaporation onto a solid substrate can also give 2-D arrangements, andthis invention should not be seen as limited in its arrangement methods.

EXAMPLE 1

[0035] This example illustrates the preparation of apoferritin fromhorse spleen ferritin. Apoferritin was prepared from cadmium-free nativehorse spleen ferritin (CalBiochem, 100 mg/ml) by dialysis (molecularweight cut-off of 10-14 kDaltons) against sodium acetate solution (0.2M) buffered at pH 5.5 under a nitrogen flow with reductive chelationusing thioglycolic acid (0.3 M) to remove the ferrihydrite core. This isfollowed by repeated dialysis against sodium chloride solution (0.15 M)to completely remove the reduced ferrihydrite core from solution.

EXAMPLE 2

[0036] This example illustrates the preparation of cobalt metal withinapoferritin. The apoprotein is added to a deaerated TES/sodium chloridesolution (0.1/0.4 M) buffered at pH 7.5 to give an approximate 1 mg/mlworking solution of the protein. A deaerated cobalt(II) [for example, asthe acetate salt] solution (1 mg/ml) was added incrementally such thatthe total number of atoms added was approximately 500 atoms/apoproteinmolecule. This was allowed to stir at room temperature for one day in aninert atmosphere. This is followed by reduction of the cobalt(II) saltwith sodium borohydride to cobalt(0) metal. The final product yielded asolution of cobalt particles, each surrounded by a ferritin shell.

EXAMPLE 3

[0037] This example illustrates the preparation of a metal alloy such asyttrium cobalt (YCo₅) within apoferritin. The metal alloy follows thesame procedure as Example 2 but using a 1:5 ratio of yttrium(III) [forexample, as the acetate salt] to cobalt(II) [for example, as the acetatesalt]. The final product yielded a solution of yttrium cobalt particles,each surrounded by a ferritin shell.

EXAMPLE 4

[0038] This example illustrates the preparation of a metal ferrite suchas cobalt ferrite (CoO·Fe₂O₃) within apoferritin. The apoprotein isadded to a deaerated MES/sodium chloride solution (0.1/0.4 M) bufferedat pH 6 to give an approximate 1 mg/ml working solution of the protein.A deaerated solution of cobalt(II) [for example, as the acetate salt]and iron(II) [for example, as the ammonium sulphate salt] in a ratio of1:2 is added incrementally and allowed to air-oxidise. The final productyielded a solution of cobalt ferrite particles, each surrounded by aferritin shell.

EXAMPLE 5

[0039] This example illustrates the 2-D arrangement of ferritin-encasedmagnetic particles. An aqueous solution of particles [from Examples 2-4,and whose uniformity in size has been selected] is injected into anMES/glucose subphase solution (0.01 M/2%) contained in a Teflon trough.The particles spread at the air-subphase interface, and a portiondenature to form a monolayer film. The 2-D arrangement of encasedparticles occurs underneath this monolayer. After 10 minutes at roomtemperature, the arrangement and monolayer are transferred to asubstrate by placing the substrate directly onto the monolayer for 5minutes. After withdrawing the substrate, the attached arrangement iscoated with a thin layer of carbon for protection.

1. A magnetizable device which comprises a magnetic layer composed ofdomain-separated, ferromagnetic particles each of which has a largestdimension no greater than 100 nm.
 2. Magnetic recording medium whichincludes a magnetizable layer thereon, wherein said magnetizable layercomprises a plurality of ferromagnetic particles each having a largestdimension no greater than 100 nm, and each of which particles representsa separate ferromagnetic domain.
 3. Magnetic recording medium accordingto claim 2, wherein the distance between adjacent ferromagnetic domainsis at least 2 nm.
 4. Magnetic recording medium according to claim 2 or3, wherein the distance between adjacent ferromagnetic domains is nogreater than 10 nm.
 5. Magnetic recording medium according to claim 1,2, 3 or 4, wherein each ferromagnetic particle is encased within anorganic macromolecule.
 6. Magnetic recording medium according to claim5, wherein each ferromagnetic particle is encased within the cavity oropening of a protein macromolecule.
 7. Magnetic recording mediumaccording to claim 6, wherein each ferri- or ferromagnetic particle isencased within an apoferritin protein.
 8. A magnetic compositioncomprising a plurality of ferromagnetic particles each of which is boundto an organic macromolecule, and each of which ferromagnetic particleshas a largest dimension no greater than 100 nm.